This invention pertains in general to nuclear reactor fuel assemblies and in particular to a new thermal expansion compensation system for such assemblies.
The number of fuel elements which are used to form the reactive region in a nuclear reactor is ordinarily determined by the critically necessary mass of fissile material and by other considerations, such as the desired energy output and the allowable thermal character of the region. Conventionally, the fuel elements are formed into bundles or sub-assemblies, with the sub-assemblies being assembled or combined to form an overall assembly or reactive region.
The spaced fuel elements located within the same bundle or sub-assembly can experience varying rates of heat generation resulting in differing rises in temperature. Moreover, such factors as flux peaking in adjacent coolant channels, unequal distribution of coolant flow through the core region, presence of adjacent structural material, xenon-tilt and other flux perturbations, also lead to the same effect. Accordingly, the spaced fuel elements respond with correspondingly different thermal expansions or contractions so that, unless means are provided for offsetting this thermal effect, the bundle will be subjected to deformation or bowing, which, in general, is undesirable since "hot spots" or regions of extreme temperature rise in the fuel elements can than result and removability of the fuel bundle is impaired. An additional undesirable effect arises when peripherally located fuel elements bow to jam or obstruct control rod movement.
The aforementioned problems experienced in the boiling water reactors and pressurized water reactors is amplified in the liquid metal fast breeder reactors where the temperature gradients in different parts of the core are even more extreme. The different temperatures expected for the various sections of the liquid metal breeder reactor will cause different thermal expansions among these various sections. Thus, components made of the same material which are aligned at room temperature will not be aligned at operating conditions. For instance, control rods, their drive mechanism and the guide tubes in the reactor core will not remain aligned when the core support plate is exposed to inlet sodium at approximately 750.degree.F, the top of the core assemblies are exposed to outlet sodium at approximately 1000.degree.F and the head, to which the control rod drive mechanism is fastened, is maintained at approximately 400.degree.F. This misalignment causes control rods to bind and not insert when required, thus creating a serious control problem.
The heat generated in the fuel is highest in the center of the core and decreases away from the center. This descrease in heat generation causes a corresponding decrease in the temperature of the fuel, the cladding and the assemblies. Thus a given part of the core will have a higher temperature on the side toward the center than on the outside, causing a temperature gradient through the part. This change in temperature through the part causes uneven thermal expansion with the side closest to the center expanding more, because it is hotter. The uneven thermal expansion causes the part to bend or bow as mentioned above. The thermal bowing in turn causes the fuel sub-assemblies to move toward the center of the core, creating an unstable nuclear characteristic, due to the increased concentration of fissile material, which presents control problems.
The problem becomes extremely critical when the movement of the fuel sub-assembly towards the center of the core occurs in a short period of time. This might occur if a subassembly is twisted slightly and pushes against a neighboring sub-assembly. The increasing bowing force will sooner or later overcome the friction between the sub-assemblies, or a random vibration will trigger the movement. Then, the sub-assembly will bend very rapidly towards the center. Even worse, a chain reaction could be caused thereby where several subassemblies jump one after the other. This motion of subassemblies towards the center increases the effective heavy metal mass density with a resultant increase in power, possibly to a dangerous level.
The prior art has been able to minimize the problems caused by the uneven thermal expansion of the various components within the reactor by utilizing different structural support designs in the fuel assembly as illustrated by application No. 19,851 entitled "FUEL ARRANGEMENT FOR A NUCLEAR REACTOR" filed Apr. 4, 1960, now abandoned, and assigned to the Westinghouse Electric Corporation. Another support method was illustrated in application No. 19,760 entitled "MEANS FOR SUPPORTING FUEL ELEMENTS IN A NUCLEAR REACTOR" filed Apr. 4, 1960, now U.S. Pat. No. 3,182,003, and assigned to the Westinghouse Electric Corporation. While the techniques presented in the aforementioned applications for Letters Patent have been able to overcome the problems caused by the thermal gradients in the pressurized water reactor and boiling water reactor, it is not expected that they will be able to solve problems caused by the extreme temperature graidents anticipated in the liquid metal fast breeder reactors.